This is purposely sort of a troll post, but I'm trying to bring out the technical minds to discuss this.
The simple question:
What is the danger in having valve closing clearances on a desmodromic engine that are too tight?
Some points I thought up, but I can't make them all work together:
o What is the thermal growth difference between the valve itself (steel) and the distance between the valve seat and the cam (aluminum)? (does it matter for closing clearance since the closing rocker isn't directly above the valve as far as actuation? maybe it matters only for opening clearance?)
o What is the thermal growth (thickness) of the rocker? (does it matter?)
o Why is it that on some engines the spec is close to zero, and on some it's much higher, up to maybe .008" at the high side of the tolerance?
In my pea shaped brain, it seems to me that the half rings and valves would be much happier with less closing clearance, the least possible, to prevent thrashing of those half rings. Maybe this is an overly simplistic view?
__________________
'01 996 (ASMA #17, rebuild complete, runs, needs more stuff as it goes with Ducatis)
'01 MV F4S
'02 998 www.asmaracing.com www.desmosouthwest.com
Well the expansion rate of steel is 8 millionths of an inch per degree F. Aluminum is,,, aww heck, why bother. The rockers obviously move to or from the cam location X distance as the head heats. The closers on Testastrettas are a 90 degree thing with a ratio of ?:1. The openers have a ratio of ??:1. If you're bored, for a 998 testastretta exhaust valve the stem is 4 1/16" from the center of the seat to the stem tip. I'm not going to calc the stem length increase with heat in it!
Aah, umm,, on a stretta, .004" works well. I agree that excessive clearance will hammer the valve train and likely be the cause of broken half rings as the valves are bouncing betwen the opener and closer rockers and cam at a high frequency during the excursion. Yet on the single cam engines the closer clearance can be almost nothing which seems most logical to me.
So why do the 4v engines have .004? Is it because the factory can't allow the time to shim them tighter so they shoot for larger values?
Running too tight may inhibit the time needed for an oil film to develop between cam and rocker. Yet again on the sohc engines this closer clearance can be almost nil. Yeah, why can't we run them tighter on both closers and openers.
I never checked clearance on my engine when it's hot. Will the clearances tighten or open up when hot? Dunno but I'll check next time I'm it as it's easy to pop the rear cover on a stretta. If the clearances diminish significantly I'll have my answer as to why .004" is the lower limit.
Perhaps a seasoned guru can weigh in on this and splain why we shouldn't run tighter on these engines.
About 14 years ago, (Fast by) Ferracci came up with the following recommendations by taking a 4V head and turning the cams with an electric motor in the 10,000 rpm region and seeing what happened. He found that these clearances minimized issues with valve bounce and float. These numbers don't have anyting to do with running clearance or heat or anything, they're just what seemed to work the best mechanically.
Eraldos test was a good start. Sounds like he didn't get any help from the factory on the subject. As the old every reaction has an equal and opposite reaction holds, I'd like to see another study of the phenomena with high speed photography.
As the opener clobbers the valve it is accelerated downwards where it contacts the closer. Which boots it back up against the opener. Repeat this X times for each valve excursion. Then the valve gets the seat bounce and radial stem wiggle before it settles down for the compression cycle. There is a lot of violent physics at work here!
I 'm not going to provide the tight clearance test mule but my logic follows that if there is a couple thousandths clearance when the engine is cold and a couple thousandths when hot that all would be good. As an engineering study one would keep reducing clearances until something got damaged. Rockers and cam scuffing or other maladies. Then open clearances until reliability is insured. Hmm, maybe that .004" is correct.
Until I can measure hot clearance I would logically assume that clearances will change when hot as there is a lot of physical distance of aluminum between the valve seat and the cam bearings. Considering this, perhaps closers will tighten a bit and openers slightly less as the aluminum expands.
Aluminum expands .000023 mm per degree Celsius depending on the alloy. A 1 meter bar will grow 2.3 mm if its temperature increases 100 degrees C.
Something fun to watch. The mass off the valve spring knocks the valve around quite a bit. Desmos have the bounce between openers and closers.
About 14 years ago, (Fast by) Ferracci came up with the following recommendations by taking a 4V head and turning the cams with an electric motor in the 10,000 rpm region and seeing what happened. He found that these clearances minimized issues with valve bounce and float. These numbers don't have anyting to do with running clearance or heat or anything, they're just what seemed to work the best mechanically.
Intake
Opener - 0.10mm
Closer - 0.10mm
Exhaust
Opener - 0.15mm
Closer - 0.10mm
Do you know for what engine this is? It would have to be either air cooled or a Desmoquattro of course...
The reason I ask is because the length of the rocker must also play a role in this, depending on lift and acceleration rate of the cam.
I had a brief discussion about this with Kevin Cameron at the WSB race at Miller. He said that the Ducati engineers are aware the there is a spring effect of the steel rockers, and this does play a role with design of the engines, including the MotoGP engine.
With this much clearance, there must be something more going on that just minimizing clearance...
__________________
'01 996 (ASMA #17, rebuild complete, runs, needs more stuff as it goes with Ducatis)
'01 MV F4S
'02 998 www.asmaracing.com www.desmosouthwest.com
Nice video OF. That really paints a nice picture of what's really going on, although I'm sure one could theorize that all this stuff happens... To see it is pretty amazing.
__________________
'01 996 (ASMA #17, rebuild complete, runs, needs more stuff as it goes with Ducatis)
'01 MV F4S
'02 998 www.asmaracing.com www.desmosouthwest.com
I never checked clearance on my engine when it's hot. Will the clearances tighten or open up when hot? Dunno but I'll check next time I'm it as it's easy to pop the rear cover on a stretta. If the clearances diminish significantly I'll have my answer as to why .004" is the lower limit.
Perhaps a seasoned guru can weigh in on this and splain why we shouldn't run tighter on these engines.
I've done some testing with the desmoquattro's in this arena. What I've learned is; everything tightens up when hot. I've seen .001" on average and in some cases, even slightly more. Also, the desmoquattro's cam's flex much more then the testastretta's do this means, the clearances on the desmoquattro's need to be a bit looser to help compensate for that bending effect. Also, tighter clearances, really don't have any benefit. I've done tests with .002" closing/opening clearance and it didn't change anything vs .005" opening/closing clearance. I've recently done leak-down tests and compression tests on good heads trying to determine if it really makes a difference and in all honesty, my testing concluded; proper belt tensioning makes MORE of a difference then clearances.
Needless to say the testastretta with its center cam holding position, helps greatly with cam flex, throw some MBP's in there and you should theoretically be able to run pretty tight clearances to help avoid the rattling around effect.
Well, my guess is that it's a warm up transitional issue. The valves will heat up first but more to the point is that the head of the valve will heat up faster then the stem. That being the case, I would think the clearance gets tighter before getting looser. The 2 valve engines may have enough surface area at the seat to head to dissipate heat quicker so may not need as much clearance as the 4 valvers. Just my guess.
I've tried very tight closing specs on both quattro and testa with good result, although using MBP halfrings on all of them.
A couple of hundred of millimeters would do it, like you wouldnt fit the thinnest of guages in there, took it apart after one season and its all good.
I tried warming up one of the engines real good and then unhook the belt and it was still lean and the after-season result confirmed that it was good, no wearing.
This is what I expected so far. It seems like the closing clearance can only open up with heat since the valve will grow, but the growth of the head from the seat to the cam shouldn't be an issue due to the 90 degree closing rocker (I'm speaking of the 4v motors, I've never been in a 2v one).
A guy I used to race with that had an air cooled supersport said that he used to set the closers tight enough so that none of his feeler gauges would fit in the gap, but he could spin the shim with his finger. Effectively 0 clearance, but no binding.
__________________
'01 996 (ASMA #17, rebuild complete, runs, needs more stuff as it goes with Ducatis)
'01 MV F4S
'02 998 www.asmaracing.com www.desmosouthwest.com
Seems logical that the 4V engines can run with tight clearances as the 2V does. In my primitive mind the reduction of clearances must be a better method of control and enhance half ring reliability. Not that there is a problem in that area with the testastrettas.
I'll sit back and watch the thread for a while before going for that .0015 feeler gage.
Quote: It seems like the closing clearance can only open up with heat since the valve will grow, but the growth of the head from the seat to the cam shouldn't be an issue due to the 90 degree closing rocker
This is what I’m thinking. The head of the valve grows first as it is closest to the flame. As it gets bigger in diameter the valve would move towards the combustion chamber decreasing clearance even though the overall length of the valve gets longer. The key here is the head is growing in diameter so it pushes away from the seat. Think of it as the opposite of valve recession where the valve sinks into the head.
If the seat grew in diameter at the same rate it would cancel out.
As the stem warms up and gets longer clearance comes back. I don’t see how the shape of the rocker plays into it.
Valve closing clearance question/discussion
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